Modelling the evolution of Arctic melt ponds

Abstract

During winter the ocean surface at the poles freezes over to form sea ice. Sea ice floats
on the ocean surface and has a matrix structure caused by the rejection of salts during
freezing. In the summer sea ice melts at its surface creating melt ponds.
An accurate estimate of the fraction of the upper sea ice surface covered in melt
ponds during the summer melt season is essential for a realistic estimate of the albedo for
global climate models. I will present a melt-pond{sea-ice model that simulates the twodimensional
(areal) evolution of melt ponds on an Arctic sea-ice surface. This advancements
of this model compared to previous models are the inclusion of snow topography,
a realistic hydraulic balance and calculation of drainage rates and the incorporation of
a detailed one-dimensional thermodynamic model. Water transport across and through
the sea-ice surface is described by the major hydraulic processes believed to be present.
Thermodynamic processes are modelled using the mushy-layer equations in sea ice, heat
diff�usion equations in snow and using assumptions of turbulent heat flux in melt ponds,
along with a three-layer two-stream radiation model.
The model simulates a section of a sea ice
floe considered to be in hydrostatic equilibrium,
where edge eff�ects such as the presence of leads are neglected and consists of a
grid of cells, each of which can be in one of four possible con�figurations: snow covered
ice; bare ice; melt pond covered ice or open water. Eventually, a cluster of adjacent cells
each containing melt water may be considered to have formed a melt pond. Lateral and
vertical melt water transport is described by Darcy's Law.
The model is initialised with ice topographies that represent either �first-year or multiyear
sea ice, which are reconstructed from SHEBA ice thickness data using standard
statistical methods. The roughness and thickness of the ice and snow surfaces were altered
and the sensitivity of the model to the initial data was tested. First-year ice and multiyear
ice simulations con�firmed observed diff�erences in individual pond size and depth.
Sensitivity studies showed that pond fraction is most sensitive to mean initial snow depth
in fi�rst-year ice simulations and reduction of ice permeability in all cases.